Can I find a partner? Undecidability of partner existence for open nets

We study open nets as Petri net models of web services, with a link to the practically relevant language WS-BPEL. For those nets, we investigate the problem of operability which we consider as fundamental as the successful notion of soundness for workflow nets, i.e., Petri net models of business processes and workflows. While we could give algorithmic solutions to the operability problem for subclasses of open nets in earlier work, this article shows that the problem is in general undecidable.     

Case-based adaptation of workflows

This paper presents on a Case-based Reasoning approach for automated workflow adaptation by reuse of experience. Agile workflow technology allows structural adaptations of workflow instances at build time or at run time. The approach supports the expert in performing such adaptations by an automated method. The method employs workflow adaptation cases that record adaptation episodes from the past. The recorded changes can be automatically transferred to a new workflow that is in a similar situation of change. First, the notion of workflow adaptation cases is introduced. The sample workflow modeling language CFCN is presented, which has been developed by the University of Trier as a part of the agile workflow management system Cake. Then, the retrieval of adaptation cases is briefly discussed. The case-based adaptation method is explained including the so-called anchor mapping algorithm which identifies the parts of the target workflow where to apply the changes. A formative evaluation in two application domains compares different variants of the anchor mapping algorithm by means of experts assessing the results of the automated adaptation.     

DFL: A dataflow language based on Petri nets and nested relational calculus

In this paper we propose DFL—a formal, graphical workflow language for dataflows, i.e., workflows where large amounts of complex data are manipulated, and the structure of the manipulated data is reflected in the structure of the workflow. It is a common extension of (1) Petri nets, which are responsible for the organization of the processing tasks, and (2) nested relational calculus, which is a database query language over complex objects, and is responsible for handling collections of data items (in particular, for iteration) and for the typing system. We demonstrate that dataflows constructed in a hierarchical manner, according to a set of refinement rules we propose, are semi-sound, i.e., initiated with a single token (which may represent a complex scientific data collection) in the input node, terminate with a single token in the output node (which represents the output data collection). In particular they never leave any “debris data” behind and an output is always eventually computed regardless of how the computation proceeds.     

Workflow performance analysis and simulation based on multidimensional workflow net

Workflow model performance analysis plays an important role in the research of workflow techniques and efficient implementation of workflow management. Instances dwelling times (IDT) which consist of waiting times and handle times in a workflow model is a key performance analysis goal. In a workflow model the instances which act as customers and the resources which act as servers form a queuing network. Multidimensional workflow net (MWF-net) includes multiple timing workflow nets (TWF-nets) and the organization and resource information. This paper uses queuing theory and MWF-net to discuss mean value and probability distribution density function (PDDF) of IDT. It is assumed that the instances arrive with exponentially distributed inter-arrival times and the resources handle instances within exponentially distributed times or within constant times. First of all, the mean value and PDDF of IDT in each activity is calculated. Then the mean value and PDDF of IDT in each control structure of a workflow model is computed. According to the above results a method is proposed for computing the mean value and PDDF of IDT in a workflow model. Finally an example is used to show that the proposed method can be effectively utilized in practice.     

Computing maximal Kleene closures that are embeddable in a given subword-closed language

Motivated by certain coding techniques for reliable DNA computing, we consider the problem of characterizing nontrivial languages D that are maximal with the property that D ^* is contained in the subword closure of a given set S of words of some fixed length k. This closure is simply the set of all words whose subwords of length k must be in S. We provide a deep structural characterization of these languages D, which leads to polynomial time algorithms for computing such languages.     

Cops and Robbers from a distance

Cops and Robbers is a pursuit and evasion game played on graphs that has received much attention. We consider an extension of Cops and Robbers, distance k Cops and Robbers, where the cops win if at least one of them is of distance at most k from the robber in G. The cop number of a graph G is the minimum number of cops needed to capture the robber in G. The distance k analogue of the cop number, written c_k(G), equals the minimum number of cops needed to win at a given distance k. We study the parameter c_k from algorithmic, structural, and probabilistic perspectives. We supply a classification result for graphs with bounded c_k(G) values and develop an O(n^2s+3) algorithm for determining if c_k(G)≤s for s fixed. We prove that if s is not fixed, then computing c_k(G) is NP-hard. Upper and lower bounds are found for c_k(G) in terms of the order of G. We prove that

     

Software architectures to integrate workflow engines in science gateways

Science gateways often rely on workflow engines to execute applications on distributed infrastructures. We investigate six software architectures commonly used to integrate workflow engines into science gateways. In tight integration, the workflow engine shares software components with the science gateway. In service invocation, the engine is isolated and invoked through a specific software interface. In task encapsulation, the engine is wrapped as a computing task executed on the infrastructure. In the pool model, the engine is bundled in an agent that connects to a central pool to fetch and execute workflows. In nested workflows, the engine is integrated as a child process of another engine. In workflow conversion, the engine is integrated through workflow language conversion. We describe and evaluate these architectures with metrics for assessment of integration complexity, robustness, extensibility, scalability and functionality. Tight integration and task encapsulation are the easiest to integrate and the most robust. Extensibility is equivalent in most architectures. The pool model is the most scalable one and meta-workflows are only available in nested workflows and workflow conversion. These results provide insights for science gateway architects and developers.     

Mediator approach to direct workflow simulation

This paper presents a direct workflow simulation method with which the future enactment service processes of a BPM system can be simulated directly (i.e., without a model conversion). The proposed method may easily be implemented on a commercial BPM system by plugging in a couple of software modules (no internal modification of the BPM system is required). Previous researches on workflow simulation relied mostly on conversion methods in which process definition models (PDMs) are converted to simulation models and the simulation is performed by a separate simulator. More recently, a direct workflow simulation method based on the concept of listener was proposed. However, with the listener approach, (1) some internal modification of the BPM system is required, (2) PDMs have to be modified slightly, and (3) reliable simulation is not guaranteed. The direct workflow simulation approach proposed in this paper, which we call the mediator approach, is free of these shortcomings. Moreover, the mediator approach is suitable for workflow simulation involving multiple BPM systems. In a ‘direct’ workflow simulation, (1) the work-list handler of each participant is replaced by a participant simulator, (2) simulation is carried out by the workflow engine of the BPM system, and (3) a software module called synchronization manager (mediator or listener) handles time synchronization during simulation. In this paper, the architecture and detailed logic of the mediator are described as DEVS models. The behaviors of participant simulators are also described as DEVS models. The proposed workflow simulation method has been implemented on a commercial BPM system as well as on an academic BPM system, and an illustrative workflow simulation example is provided.     

Almost k-wise independence versus k-wise independence

We say that a distribution over {0,1}ⁿ is (ε,k)-wise independent if its restriction to every k coordinates results in a distribution that is ε-close to the uniform distribution. A natural question regarding (ε,k)-wise independent distributions is how close they are to some k-wise independent distribution. We show that there exist (ε,k)-wise independent distributions whose statistical distance is at least n^O(k)·ε from any k-wise independent distribution. In addition, we show that for any (ε,k)-wise independent distribution there exists some k-wise independent distribution, whose statistical distance is n^O(k)·ε.     

Parameterizing cut sets in a graph by the number of their components

For a connected graph G=(V,E), a subset U⊆V is a disconnected cut if U disconnects G and the subgraph G[U] induced by U is disconnected as well. A cut U is a k-cut if G[U] contains exactly k(≥1) components. More specifically, a k-cut U is a (k,?)-cut if V?U induces a subgraph with exactly ?(≥2) components. The Disconnected Cut problem is to test whether a graph has a disconnected cut and is known to be NP-complete. The problems k-Cut and (k,?)-Cut are to test whether a graph has a k-cut or (k,?)-cut, respectively. By pinpointing a close relationship to graph contractibility problems we show that (k,?)-Cut is in P for k=1 and any fixed constant ?≥2, while it is NP-complete for any fixed pair k,?≥2. We then prove that k-Cut is in P for k=1 and NP-complete for any fixed k≥2. On the other hand, for every fixed integer g≥0, we present an FPT algorithm that solves (k,?)-Cut on graphs of Euler genus at most g when parameterized by k+?. By modifying this algorithm we can also show that k-Cut is in FPT for this graph class when parameterized by k. Finally, we show that Disconnected Cut is solvable in polynomial time for minor-closed classes of graphs excluding some apex graph.     

拟（h,k）存贮有限自动机的可逆性

主要研究拟（h,k）阶存贮有限自动机的延迟k步与k＋1步弱可逆性,以及它的弱逆,得到了拟（h,k）阶存贮有限自动机的延迟k步与k＋1步弱可逆的充分必要条件,并且通过所得结果可以比较简便地构造出延迟k步与k＋1步弱可逆拟（h,k）阶存贮有限自动机的延迟k步与k＋1步弱逆。     

New complexity results for the k-covers problem

The k-covers problem (kCP) asks us to compute a minimum cardinality set of strings of given length k > 1 that covers a given string. It was shown in a recent paper, by reduction to 3-SAT, that the k-covers problem is NP-complete. In this paper we introduce a new problem, that we call the k-bounded relaxed vertex cover problem (RVCP_k), which we show is equivalent to k-bounded set cover (SCP_k). We show further that kCP is a special case of RVCP_k restricted to certain classes G_x,k of graphs that represent all strings x. Thus a minimum k-cover can be approximated to within a factor k in polynomial time. We discuss approximate solutions of kCP, and we state a number of conjectures and open problems related to kCP and G_x,k.     

Spanners of bounded degree graphs

A k-spanner of a graph G is a spanning subgraph of G in which the distance between any pair of vertices is at most k times the distance in G. We prove that for fixed k,w, the problem of deciding if a given graph has a k-spanner of treewidth w is fixed-parameter tractable on graphs of bounded degree. In particular, this implies that finding a k-spanner that is a tree (a tree k-spanner) is fixed-parameter tractable on graphs of bounded degree. In contrast, we observe that if the graph has only one vertex of unbounded degree, then Treek-Spanner is NP-complete for k?4.     

工作流模型结构化挖掘方法研究

流程挖掘是业务流程管理(business process management,BPM)研究的一项重要内容.提出了一种结构化挖掘方法,实现从事务型日志中挖掘出工作流网.该方法基于工作流模型的4种基本结构(顺序、并行、选择和循环)进行挖掘.定义了可挖掘的工作流模型--结构化工作流网(structural workflow net,SWF),从日志预处理,流程挖掘方法和合理性验证3个方面对挖掘算法进行了详细描述,证明挖掘出的工作流模型满足合理性和安全性的同时,具有可读性和容易理解的特点.     

Refined memorization for vertex cover

Memorization is a technique which allows to speed up exponential recursive algorithms at the cost of an exponential space complexity. This technique already leads to the currently fastest algorithm for fixed-parameter vertex cover, whose time complexity is O(k^1.2832k^1.5+kn), where n is the number of nodes and k is the size of the vertex cover. Via a refined use of memorization, we obtain an O(k^1.2759k^1.5+kn) algorithm for the same problem. We moreover show how to further reduce the complexity to O(k^1.2745k⁴+kn).     

A microcomputer program for the determination of the micropores distribution functions

A program in FORTRAN 77 has been created to determine the functions of micropore volume distribution over the characteristic linear dimensions for micriporous adsorbents. It has been used Dubinin-Stoeckli and McEnaney relations between the characteristic energy E₀ or the structural parameter k and half-width of micropores x.     

Two iteration theorems for the LL(k) languages

The structure of derivation trees over an LL(k) grammar is explored and a property of these trees obtained which is shown to characterize the LL(k) grammars. This characterization, called the LL(k) Left Part Theorem, makes it possible to establish a pair of iteration theorems for the LL(k) languages. These theorems provide a general and powerful method of showing that a language is not LL(k) when that is the case. They thus provide for the first time a flexible tool with which to explore the structure of the LL(k) languages and with which to discriminate between the LL(k) and LR(k) language classes.Examples are given of LR(k) languages which, for various reasons, fail to be LL(k). Easy and rigorous proofs to this effect are given using our LL(k) iteration theorems. In particular, it is proven that the dangling-ELSE construct allowed in PL/I and Pascal cannot be generated by any LL(k) grammar. We also give a new and straightforward proof based on the LL(k) Left Part Theorem that every LL(k) grammar is LR(k).     

Simulated annealing type algorithms for multivariate optimization

We study the convergence of a class of discrete-time continuous-state simulated annealing type algorithms for multivariate optimization. The general algorithm that we consider is of the formX _k +1 =X _k ?a _k (▽U(X _k ) + ξ_k) +b _k W _k . HereU(·) is a smooth function on a compact subset of ? ^d , {ξ_k} is a sequence of ? ^d -valued random variables, {W _k } is a sequence of independent standardd-dimensional Gaussian random variables, and {a _k }, {b _k } are sequences of positive numbers which tend to zero. These algorithms arise by adding decreasing white Gaussian noise to gradient descent, random search, and stochastic approximation algorithms. We show under suitable conditions onU(·), {ξ _k }, {a _k }, and {b _k } thatX _k converges in probability to the set of global minima ofU(·). A careful treatment of howX _k is restricted to a compact set and its effect on convergence is given.